Studies of cell surface markers for rabbit lymphoid cell populations have emphasized rabbit T cells with special reference to those involved in retrovirus infection. The rabbit CD4 (RbCD4) gene was used in transfection experiments to compare its efficacy as a receptor for HIV-I to human CD4 which is known to bind the env protein of HIV with high affinity. RbCD4 transfectants of human and mouse cell lines were compared with these lines expressing human CD4. RbCD4 proved to be a poor receptor for HIV-1 and therefore rabbits transgenic for human CD4 were developed. Transgenic animals have been obtained and shown to express HuCD4 on appropriate T lymphocyte populations and in a development specific fashion. Infections of PBMC from the transgenics with HIV-1 are more productive than normal rabbit infection. Preliminary data indicate that PBMC from the transgenic rabbits undergo apoptosis in the presence of cell free HIV-I whereas no effect on normal rabbit PBMC is noted. Several transgenic rabbit lines are being bred for study. Other rabbit lymphoid cell marker investigated include the T cell receptors gamma delta, CD8beta and IL-2rapha. Transcripts of the delta TCR gene occurred in two distinct forms, a duplication of exon 2 of the constant region was found in the larger message. This was shown to result from a genetic polymorphism in which the gene encoding TCR Cdelta was duplicated along with some flanking intron sequences. This insertion deletion polymorphism (IDRP) was shown to be inherited as an autosomal codominant trait and animals homozygous for the inserted form had levels of gamma delta T cells in their peripheral blood that were equal to rabbits homozygous for the shorter form. Present studies continue to characterize T cell surface markers that may play a role in pathogenic effects of HTLV-I infected cells.